10 December 2010

A prime objective of the Cassini orbital mission at Saturn is to characterize the nature and evolution of Saturn's extended family of icy satellites. Cassini observations since the beginning of the prime mission in mid-2004 have made possible the first global maps of these diverse bodies. A team of scientists lead by Dr. Paul Schenk at the Lunar and Planetary Institute in Houston have produced the first global color and topographic maps of these satellites. These two views of Tethys show the high-resolution color (at left) and the topography (at right) of the leading, or forward-facing, hemisphere of this 1060-km-diameter ice-rich satellite. The color map shows the prominent dusky bluish band along the equator, first seen by Voyager in 1980, and shown by the team of scientists lead by Dr. Schenk to be due to the bombardment and alteration of the surface by high energy electrons traveling slower than the satellite's revolution period. These findings were published in the journal Icarus (see previous posts). The general reddish tones may be due to the coating of the Tethyean surface by dust-sized particles ejected by Enceladus' south polar plumes. The view at right is a color-coded topographic map of the same region (blues are low, reds are high). The total range of topography shown is 10 kilometers from highest to lowest point. The dominant feature is the 8-kilometer-deep and 440-kilometer-wide Odysseus impact basin at upper left. Straddling the view like a belt is a previously unknown topographic ridge between 2 and 3 kilometers high. To the east of the ridge lies ordinary rolling cratered plains, but between the ridge and Odysseus the surface is scoured and pockmarked. The ridge may be a tectonic feature related to the impact event or may be a deposit formed when the ejecta blasted out of Odysseus slammed back onto the surface at high velocity. At bottom right can be seen the globe-circling trench Ithaca Chasma, formed as part of Tethys stretched apart. This map is part of a set of new global topographic maps produced by Dr. Schenk's team for each of Saturn's icy satellites and previewed on-line on this very blog (see previous post)!

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NOTES ABOUT THE DATA

These brief notes are posted to help viewers understand the nature of the topographic data. Voyager, Galileo and Cassini carried imaging cameras but not altimeters. Topography is instead generated using stereo images or low-Sun images (which can be used to calculate slopes and heights). Neither method is perfect and often, as is the case for Miranda, individual stereo pairs must be constructed and then stitched back together to form a global or partial topographic map. This means that the elevations shown are not precise with respect to the center of the body. Height values derived are accurate however with respect to local features. For example, we know the steep cliff on Miranda is about 10 km high top to base but we do not know how high it is with respect to the mean "sea level" on Miranda. This cannot be determined until we return to these places.I will post additional information on the data over the next few days.

Inquiries for the scientific use of the original digital elevation data should be forwarded to schenk@lpi.usra.edu.